Before the 20th century, it was generally believed in biology that all physiological activities of human beings and animals were regulated by the nervous system. French scholar Voltmer divided the pancreas. (1) The basic way of nerve regulation is reflex, and the structural basis of reflex is reflex arc, which consists of five parts, namely receptor → afferent nerve → nerve center → efferent nerve → effector.

(2) Through the analysis of the stem, we can know that ① plays a controlling role in Votaimer's experimental design. Comparing the experiments of ① and ② in stem, it shows that dilute hydrochloric acid must stimulate intestinal mucosa to make pancreas secrete pancreatic juice. Comparing the experiments of ① and ③ in stem, we can know that the independent variable is whether there are small intestinal nerves, and the dependent variable of the experiments of ①, ② and ③ is whether the pancreas secretes pancreatic juice.

(3) According to the body fluid, the purpose of this experiment is to verify the hypothesis that "under the action of hydrochloric acid, a chemical produced by intestinal mucosa causes pancreatic juice secretion". The experimental steps are as follows: mixing small intestinal mucosa with dilute hydrochloric acid, grinding into gravel to make extract, injecting it into dogs' veins, and observing whether the pancreas secretes pancreatic juice; They call this substance a substance secreted by small intestinal mucosa under the action of glucagon hydrochloride; This regulation is not nerve regulation, but hormone regulation, because it excludes the function of nerve.

(4) As can be seen from the meaning of the question, protein is a secretin, and its transmembrane transport mode is exocytosis. Because trypsin can decompose protein on the surface of cell membrane and disperse animal tissues and cells.

So the answer should be:

(1) reflection? Reflex arc receptor → afferent nerve → nerve center → efferent nerve → effector.

(2) comparison? Must stimulate intestinal mucosa? Whether the pancreas secretes pancreatic juice.

(3) intravenous injection of dilute hydrochloric acid in dogs? Humoral regulation (hormone regulation)

(4) exocytosis? trypsin